An asynchronous motor operated on the AC mains supply needs at least two phase-shifted voltages in order to create a rotating field. For this purpose, the asynchronous motor normally has a main winding and an auxiliary winding offset by 90°, the phase shift being achieved by a capacitor connected in series with the auxiliary winding.
The phase shift is a function of the capacitance of the capacitor and the speed of the asynchronous motor and determines its quiescent point. Operation at the incorrect quiescent point can lead to malfunctions and damage to the components or the machinery assigned thereto. It is therefore often necessary to perform a change to the capacitance of the capacitor in the start-up phase of the motor between the start-up and the nominal speed, the change in capacitance normally being achieved as follows:    1. The asynchronous motor is operated with a start capacitor and a run capacitor, the start capacitor being disconnected after the starting procedure, or    2. the asynchronous motor is operated with a start capacitor only, which is disconnected after the starting procedure.
The available torque must remain significantly greater than the load torque both before and immediately after the disconnection of the start capacitor for failure-free motor operation. In practice, it is standard to bring about the disconnection of the start capacitor using a centrifugal switch dependent on the motor speed, using a current relay dependent on the motor current, or after a predefined time.
Furthermore, a method is known from US 2007/0057647 A1, in which the phase shift between the voltage of the auxiliary winding and the voltage of the main winding is determined and the start capacitor is disconnected when the phase shift exceeds a specified value.
Moreover, a motor circuit for a single-phase alternating current induction motor, comprising a main winding, an auxiliary winding and a start capacitor, is known from DE 600 24 487 T2, the voltage of the auxiliary winding being measured and the disconnection of the start capacitor being initiated when the first derivative function of the measured voltage exceeds a specified threshold value.